1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, and a process cartridge and an electrophotographic apparatus including an electrophotographic photosensitive member.
2. Related Background Art
Recently an electrophotographic photosensitive member utilizing an organic photoconductive material (organic electrophotographic photosensitive member) is being actively developed.
An electrophotographic photosensitive member is basically constituted of a substrate, and a photosensitive layer formed on such substrate. In practice, however, various layers are often formed between the substrate and the photosensitive layer, for the purposes of covering a defect on the surface of the substrate, improving a coating property of the photosensitive layer, improving adhesion between the substrate and the photosensitive layer, protecting the photosensitive layer from electrical destruction, improving a charging property, and improving a charge injecting property from the substrate to the photosensitive layer. Therefore, for a layer provided between the substrate and the photosensitive layer, various functions are required such as a covering property, an adhesion property, mechanical strength, electroconductivity and an electrical barrier property.
The layer provided between the substrate and the photosensitive layer has been known to be following types:    (i) a resin layer not containing a conductive material;    (ii) a resin layer containing a conductive material; and    (iii) a layer (i) laminated on a layer (ii) mentioned above.
The aforementioned layer (i) has a high electrical resistance as it does riot contain a conductive material. Also it has to be provided with a large thickness (film thickness) in order to cover a defect on the substrate surface not subjected to a surface smoothing process.
However, the aforementioned layer (i) of a high electrical resistance, when provided with a large film thickness, results in a drawback of a high residual potential in an initial state of use and after repeated use.
Therefore, for practical use of the layer (i), it is necessary to reduce the defects on the substrate surface and to reduce the film thickness.
On the other hand, the aforementioned layer (ii), being formed by dispersing a conductive material such as conductive particles in a resin and capable of reducing the resistance of the layer, can be employed with a large film thickness thereby covering a surface defect of an electroconductive substrate or a non-conductive substrate (such as a resinous substrate).
However, in case of increasing the thickness of the aforementioned layer (ii), it is necessary, in comparison with the layer (i) which is made thinner, to provide the layer with a sufficient electrical conductivity, whereby the layer (ii) will have a low volume resistivity. For this reason, in order to avoid a charge injection from the substrate or the layer (ii) into the photosensitive layer, constituting a cause of an image defect, over wide environmental conditions from a condition of a low temperature and a low humidity to a condition of a high temperature and a high humidity, it is preferable to provide a layer having an electrical barrier property between the layer (ii) and the photosensitive layer. The layer having an electrical barrier property is, for example, a resin layer not containing conductive particles such as the aforementioned layer (i).
Stated differently, the layer provided between the substrate and the photosensitive layer preferably has an aforementioned configuration (iii), formed by lamination of the layer (i) and the layer (ii).
The configuration (iii) involves an increased number of process steps since plural layers have to be formed, but increases a tolerance for the surface defect of the substrate, thereby significantly widening the tolerance for the substrate and providing an advantage of increasing the productivity.
In general, the aforementioned layer (ii) is called a conductive layer, and the layer (i) is called an intermediate layer (undercoat layer or barrier layer).
A conductive material to be employed in the conductive layer includes various metals, metal oxides and conductive polymers. Among these, tin oxide (SnO2) having excellent resistance characteristics is known as conductive materials of various types such as an ordinary compound with a powder resistivity of 104-106 Ω·cm, a compound which is mixed (doped), at the manufacture of SnO2 conductive material, with a compound of a metal of a valence number different from that of tin, such as antimony oxide or a non-metal element for reducing the powder resistivity to 1/1000-1/100,000, and a non-doped oxygen-deficient SnO2 in which the resistance of SnO2 is reduced to a case of antimony doping without increasing the constituent elements.
As a prior technology relating to the oxygen-deficient SnO2, Japanese Patent Application Laid-Open No. H07-295245 discloses a technology of employing the oxygen-deficient SnO2 in the conductive layer, also Japanese Patent Application Laid-Open No. H06-208238 discloses a technology of coating barium sulfate particles with oxygen-deficient SnO2 for improving the dispersibility in comparison with a case of employing SnO2 only, and Japanese Patent Application Laid-Open No. H10-186702, though not disclosing an embodiment of oxygen-deficient SnO2, discloses a technology of employing barium sulfate particles for improving the dispersibility, coating titanium oxide (TiO2) thereon for improving the whiteness, and further coating SnO2 thereon for providing the electric conductivity.
Also because of a recent improvement in the charging uniformity of a charging apparatus, the necessity for charge pre-elimination means (such as a pre-exposure apparatus) for preventing a charging unevenness from an output image is decreasing, and there is being requested an electrophotographic apparatus of a configuration without such charge pre-elimination means in view of space saving and cost reduction.
However, in case the charge pre-elimination means such as the pre-exposure apparatus is dispensed with, a ghost image of a rotation cycle of the electrophotographic photosensitive member (a phenomenon in which an exposure hysteresis (such as a solid black image) in a one-rotation-preceding cycle of the electrophotographic photosensitive member appears on halftone image) becomes conspicuous.
The cause for such ghost phenomenon is considered the stagnation in the flow of charges (carriers) in the formation of an electrostatic latent image on the electrophotographic photosensitive member, and, in a configuration including a conductive layer, the flow of charge (carriers) tends to become stagnant because of a larger number of layers in comparison with a configuration without the conductive layer.
As it is possible, up to now, to substantially eliminate the ghost phenomenon by providing the charge pre-elimination means such as the pre-exposure apparatus and by uniformly reducing the surface potential of the electrophotographic photosensitive member before charging, the ghost phenomenon has little been raised as a technical issue. Stated differently, a fact that the ghost phenomenon becomes conspicuous in the configuration without the charge pre-elimination means such as the pre-exposure apparatus is found only recently.
As a prior technology for improving the ghost phenomenon by the structure of the conductive layer, Japanese Patent Application Laid-Open No. H07-271072 discloses a technology of employing a conductive material formed by TiO2 particles coated with SnO2 of which powder resistivity is reduced by antimony oxide doping and increasing a content of the conductive material in order to achieve a smooth flow of the charge (carriers) in the conductive layer.
However, the technology disclosed in Japanese Patent Application Laid-Open No. H07-271072, requiring an element antimony in addition to tin for coating the TiO2 particles, has a poor reuse property, and a technology utilizing oxygen-deficient SnO2, superior in the reuse property, is being expected.